feat: Phase 2 — database layer

- PostgreSQL schema: sources, manga, chapters, pages, source_meta with indexes
- golang-migrate runner with embedded SQL via go:embed (pgx5:// scheme)
- sqlc-generated type-safe queries for all tables (pgx/v5 native)
- Config package with all env vars including TTL durations
- Wire DB init and config into cmd/server/main.go

internal/db/queries/source.sql.go

@@ -0,0 +1,124 @@
+// Code generated by sqlc. DO NOT EDIT.
+// versions:
+//   sqlc v1.31.1
+// source: source.sql
+
+package queries
+
+import (
+	"context"
+)
+
+const getSourceByID = `-- name: GetSourceByID :one
+SELECT id, name, lang, is_nsfw FROM sources WHERE id = $1
+`
+
+func (q *Queries) GetSourceByID(ctx context.Context, id int64) (Source, error) {
+	row := q.db.QueryRow(ctx, getSourceByID, id)
+	var i Source
+	err := row.Scan(
+		&i.ID,
+		&i.Name,
+		&i.Lang,
+		&i.IsNsfw,
+	)
+	return i, err
+}
+
+const getSourceMeta = `-- name: GetSourceMeta :one
+SELECT value FROM source_meta
+WHERE source_id = $1 AND key = $2
+`
+
+type GetSourceMetaParams struct {
+	SourceID int64  `db:"source_id" json:"source_id"`
+	Key      string `db:"key" json:"key"`
+}
+
+func (q *Queries) GetSourceMeta(ctx context.Context, arg GetSourceMetaParams) (string, error) {
+	row := q.db.QueryRow(ctx, getSourceMeta, arg.SourceID, arg.Key)
+	var value string
+	err := row.Scan(&value)
+	return value, err
+}
+
+const listSources = `-- name: ListSources :many
+SELECT id, name, lang, is_nsfw FROM sources ORDER BY id
+`
+
+func (q *Queries) ListSources(ctx context.Context) ([]Source, error) {
+	rows, err := q.db.Query(ctx, listSources)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var items []Source
+	for rows.Next() {
+		var i Source
+		if err := rows.Scan(
+			&i.ID,
+			&i.Name,
+			&i.Lang,
+			&i.IsNsfw,
+		); err != nil {
+			return nil, err
+		}
+		items = append(items, i)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return items, nil
+}
+
+const setSourceMeta = `-- name: SetSourceMeta :exec
+INSERT INTO source_meta (source_id, key, value)
+VALUES ($1, $2, $3)
+ON CONFLICT (source_id, key) DO UPDATE
+    SET value = EXCLUDED.value
+`
+
+type SetSourceMetaParams struct {
+	SourceID int64  `db:"source_id" json:"source_id"`
+	Key      string `db:"key" json:"key"`
+	Value    string `db:"value" json:"value"`
+}
+
+func (q *Queries) SetSourceMeta(ctx context.Context, arg SetSourceMetaParams) error {
+	_, err := q.db.Exec(ctx, setSourceMeta, arg.SourceID, arg.Key, arg.Value)
+	return err
+}
+
+const upsertSource = `-- name: UpsertSource :one
+INSERT INTO sources (id, name, lang, is_nsfw)
+VALUES ($1, $2, $3, $4)
+ON CONFLICT (id) DO UPDATE
+    SET name    = EXCLUDED.name,
+        lang    = EXCLUDED.lang,
+        is_nsfw = EXCLUDED.is_nsfw
+RETURNING id, name, lang, is_nsfw
+`
+
+type UpsertSourceParams struct {
+	ID     int64  `db:"id" json:"id"`
+	Name   string `db:"name" json:"name"`
+	Lang   string `db:"lang" json:"lang"`
+	IsNsfw bool   `db:"is_nsfw" json:"is_nsfw"`
+}
+
+func (q *Queries) UpsertSource(ctx context.Context, arg UpsertSourceParams) (Source, error) {
+	row := q.db.QueryRow(ctx, upsertSource,
+		arg.ID,
+		arg.Name,
+		arg.Lang,
+		arg.IsNsfw,
+	)
+	var i Source
+	err := row.Scan(
+		&i.ID,
+		&i.Name,
+		&i.Lang,
+		&i.IsNsfw,
+	)
+	return i, err
+}